A common requirement in integrated circuit fabrication is plasma etching of openings such as contacts, vias and trenches in dielectric materials formed on semiconductor substrates. With device geometries becoming increasingly smaller, there is a requirement to form deep and narrow openings with high aspect ratios. One suitable technique for forming such openings in silicon oxide is a plasma etching technique, in which a fluorocarbon etchant gas having the general formula Cx Fy Hz, where x≧1, y≧1 and z≧0, is supplied to a process chamber of a plasma etch reactor together with one more noble gases, which perform the dual roles of providing an inert carrier gas for the etchant gas and aiding fluorine in attacking the silicon oxide.
The use of xenon as one of the noble gases has been found to provide increased selectivity and reduced resist damage in comparison to a system using argon alone. However, as xenon occurs in atmospheric air in very low concentrations, its cost is very high (the current cost of xenon is around $4/sl) and its availability can be somewhat limited. Given that the estimated usage of xenon in a plasma etch reactor comprising four processing chambers is around 250,000 to 500,000 litres per annum, it is very desirable to recover and re-use expensive noble gases such as xenon and/or krypton which are contained within the effluent stream exhaust from the process tool.
The recovery of such a noble gas, or noble gas mixture, is, however, hampered by other components of the effluent stream. These can include:                Unconsumed reactants;        By-products from the plasma etching;        Purge gas supplied to a pumping system for drawing the effluent stream from the chamber; and        Other noble gases.        
Unconsumed fluorocarbon etchant is particularly undesirable, as such gases are known to have relatively high greenhouse activity. It is relatively easy to destruct chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs) with more than one carbon atom in plasma reactors either at reduced pressure upstream from one or more vacuum pumps or at the ‘atmospheric’ exhaust of the vacuum pumps. However, CF4, due to the exceptionally strong carbon-fluorine bond, is very hard to destroy at high destruction efficiency. This is particularly a problem when exhaust gas components are recirculated back into the process and recirculation extra levels of destruction/purification are demanded to ensure the correct levels of gas purity.
Historically, CF4 is destructed by use of thermal means, for example a thermal processing unit (TPU), but these systems are expensive and not cost effective for low flows of CF4. Alternatively a plasma system can be used but here the destruction efficiency for CF4 is low; for high destruction efficiencies large, high-powered systems are required.
It is an aim of at least the preferred embodiment of the invention to provide apparatus for treating a gas stream that can remove CF4 with relatively high efficiency.